Solar cells are devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. The cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k (where 0≦x≦1, 0≦y≦1 and k is approximately 2) have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
In a thin film solar cell employing a Group IBIIIAVIA compound absorber, the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se)2 absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor vary with the Ga/(Ga+In) molar ratio. In general, for good device performance Cu/(In+Ga) molar ratio is kept at around or below 1.0. As the Ga/(Ga+In) molar ratio increases, on the other hand, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current typically may decrease. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition. It should be noted that although the chemical formula is written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation (X,Y) in chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa.
The first technique that yielded high-quality Cu(In,Ga)Se2 films for solar cell fabrication was co-evaporation of Cu, In, Ga and Se onto a heated substrate in a vacuum chamber. This technique is still popular in terms of growing absorber layers that yield high conversion efficiencies in thin film solar cell structures. However, low materials utilization, high cost of equipment, difficulties faced in large area deposition and relatively low throughput are some of the challenges faced in commercialization of the co-evaporation approach.
Another technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where at least two components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with each other and/or with a reactive atmosphere in a high temperature annealing process. For example, for CuInSe2 growth, thin layers of Cu and In are first deposited on a substrate and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction also involves sulfur, then a CuIn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer allows the growth of a Cu(In,Ga)(S,Se)2 absorber.
There are many other versions of the two-stage process that have been employed by different research groups. For example, stacked layers of sputter deposited (Cu—Ga)/In, and co-evaporated (In—Ga—Se)/(Cu—Se), and (In—Ga—Se)/Cu stacks have all been used as precursor materials which were reacted at high temperatures with S and/or Se to form the final absorber film. In two-stage processes individual thicknesses of the layers forming the stacked structure are controlled so that the two molar ratios mentioned before, i.e. the Cu/(In+Ga) ratio and Ga/(Ga+In) ratio, can be kept under control from run to run and on large area substrates.
Sputtering or evaporation techniques have been used in prior art approaches to deposit the layers containing the Group IB and Group IIIA components of the precursor stacks. In the case of CuInSe2 growth, for example, Cu and In layers were sputter-deposited on non-heated substrates and then the composite film was selenized in H2Se gas or Se vapor at an elevated temperature, as described in U.S. Pat. No. 4,798,660. Such techniques suffer from high cost of capital equipment, and relatively slow rate of production.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1A. The device 10 is fabricated on a substrate 11, such as of a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. Various conductive layers comprising Mo, Ta, W, Ti, stainless steel etc. have been used in the solar cell structure of FIG. 1A. If the substrate itself is a properly selected conductive material, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1A is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer 14a on a transparent superstrate 11a such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film 12a, and finally forming an ohmic contact to the device by a conductive layer 13a as illustrated in FIG. 1B. In this superstrate structure, light enters the device from the superstrate side.
As reviewed above, vacuum processes such as co-evaporation and sputtering are expensive techniques. With a goal of finding an inexpensive approach to absorber layer fabrication, research groups have investigated techniques comprising the steps of: i) preparing a precursor in the form of an ink or slurry containing all or some of the elemental components of Cu(In,Ga)(S,Se)2 compound, ii) depositing the ink or slurry on a substrate using methods such as spraying, doctor-blading and screen printing, to form a precursor layer, and iii) reacting the precursor layer at elevated temperatures typically with Se and/or S to from the compound film.
All of these approaches have had shortcomings. Screen printed layers, for example, did not yield high efficiency devices. Inks of metallic particles obtained by grinding larger particles, once deposited on the substrate, formed porous precursor layers. After the reaction step, these porous precursor layers yielded compound layers, which were also porous (see e.g. G. Norsworthy et al., Solar Energy Materials and Solar Cells, vol. 60, p. 127, 2000). FIG. 2 schematically shows a precursor layer 24 formed by such a prior-art method. In FIG. 2, the precursor layer 24 is deposited on the conductive film 23 on the surface of the substrate 21. The precursor layer 24 comprises particles 25, which form a matrix with voids 26 between them. The particles 25 contain at least two of the elemental components of the compound film. When such a precursor layer 24 is reacted at high temperature (e.g. 300-600° C.) with Se and S, the Cu(In,Ga)(S,Se)2 compound film forms. However, the compound film is also porous like the precursor layer. Porous compound layers present problems in terms of efficient and stable solar cell performance.
Some prior-art techniques use oxide nano-powders in the formulation of the precursor inks. For example, mixtures of copper oxide, indium oxide and gallium oxide powders or compound oxides containing Cu, In and Ga are first included in an ink formulation and then deposited on a substrate to form an oxide precursor layer. The oxide precursor layer, which contains sub-micron size oxide particles, is then annealed in a reducing atmosphere to convert oxides into metals and form a layer of multi-phase Cu—In—Ga alloys, which is then reacted with Se and/or S to form the final compound. Such an approach may reduce the porosity of the alloy layer due to the annealing step in the reducing atmosphere. However, the additional process step is costly and Ga inclusion in the layer may not be efficient because Ga-oxide is difficult to reduce even in a reducing atmosphere at temperatures of around 400° C. Furthermore, when oxide layers are reduced in a reducing atmosphere, they de-wet their substrates forming a highly non-uniform precursor layer; non-uniform in thickness and composition.
As the brief review above demonstrates, there is still need to develop low-cost deposition techniques to form high-quality dense Group IBIIIAVIA compound thin films as solar cell absorber layers.